Unidirectional pultrusion

Unidirectional, construction Refers to fibers that are oriented in the same direction (parallel alignment) such as filament-winding, pultrusion, unidirectional fabric laminate, and tape. 

Pultrusion is also used to make structural shapes from composite materials. The incoming material is generally unidirectional and must be pulled through the pultrusion die because the uncured composite material is entirely too flexible to push (as in extrusion processes). The incoming material can be preshaped by various guides and rollers as in Figure 1-18. 

Glass, carbon, and aramid fibers are used as unidirectional or fabric mat reinforcements, with E-glass/ polyester being the most commonly used system. The limitation of pultrusion is that only constant cross-section parts can be fabricated. However, a variety of hollow and solid profiles of any length can be manufactured. 

Pultrusion Close tolerance control requires care unidirectional strength... 

Continuous bundles of glass filaments are termed roving. They can be used directly in some of the most widely used fabrication procedures, such as filament winding, as well as in pultrusion (see later in the chapter for details of these techniques). The roving is unidirectional. 

The properties of composite materials cannot be predicted adequately by considering the fibre and resin constituents one by one. An important mechanism of composite failure under stress is delamination caused by differences between the engineering properties of successive plies or layers. These differences arise from the fact that successive layers may have different fibre orientations [34] or, occasionally, different fibres. It is a feature of laminates made by stacking pre-impregnated layers of reinforcement and is not an issue with, for example, unidirectional pultrusions. The process of delamination has been reviewed by Davies [35]. The fabrication of three-dimensional composites is an important step towards reducing or eliminating unwanted delaminations. Such materials are at an advanced stage of development. 

Strips consist of unidirectional fibres in the longitudinal direction of the strips that are embedded in an epoxy matrix (Fig. 4.9). They are produced in a pultrusion process. Furthermore, in addition to strips, prefabricated L-shaped strips are available (Fig. 4.10). They also have fibres only in the longitudinal direction and are embedded in an epoxy matrix. Due to their shape, however, they cannot be produced by pultrusion and therefore require a special production process. 

Graphite fibers are available to the user in a variety of forms continuous filament for filament winding, braiding, or pultrusion chopped fiber for injection or compression molding impregnated woven fabrics and unidirectional tapes for lamination. 

Anisotropic Having different properties along the three different axes (e.g. unidirectional and multi-directional laminates, filament winding and pultrusion). Opposite of isotropic. 

The impregnated tows can be used for many applications which are generally subdivided into those where the fiber reinforcement is continuous (e.g. filament winding or pultrusion) and those where the tow is cut and the unidirectionally reinforced peUets (discontinuous long fiber reinforced thermoplastics) are then further processed into parts (e.g. in extrusion or injection molding). 

Additional studies on the pultrusion of unidirectional GF/PP-oomposites have been carried out by Michaeli and coworkers (e.g. (9)). By the use of various types of hybrid yarns, these authors demonstrated... 

Tomlinson, W.J. and Holland, J.R. (1994) Pultrusion and properties of unidirectional glass fiber-polypropylene matrix composites. /. Mat. Sci. Letters, 13, 675-677. 

Devlin, B.J., Williams, M.D., Quinn, J.A. and Gibson, A.G. (1991) Pultrusion of unidirectional composites with thermoplastic matrices. Composites Manu-fact., 2(3/4), 203-207. 

A major disadvantage of pultrusion parts consisting only of unidirectional fibers is the minor strength and stiffness in cross-direction. Thus, woven fabrics can be applied to the process. In order to prevent warping of the textile, the woven fabrics have to be placed between some pull-force resistant fibers (e.g., some unidirectional fibers orientated in process direction). 

By means of the described technology properties, both processes are suitable for mass production. Nevertheless, there is a preferred field of application for both technologies. Unidirectional reinforcement fibers are best suited for the pultrusion process. In contrast, the CCM process is independent of the fiber orientation, thus it doesn t matter if unidirectional or multidirectional reinforcement materials are used. The assessment of the rates of production (= process speed) against the circumferences of the profiles, which are in contact with the tools surface (see Fig. 8.34), defines relevant process parameters and emphasizes the preferred field of application. Thus the circumference characterizes the profile shape. 

Kowsika and Mantena (1996) proposed a statistical test pattern in manufacturing unidirectional graphite-epoxy composite beams using pultrusion. The influences of significant variables of the pultrusion process together with their interactive effects on the dynamic mechanical properties were investigated. Mathematical models were subsequently derived to determine the optimal pultrusion process conditions for improved dynamic mechanical properties of the finished product. 

Fiber Orientation. Contrary to common belief, pultrusion is not Hmited to unidirectional reinforcements, but can bring reinforcing fibers into the profile in any needed orientation. 

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